For a number of years oven cavities have been employed for both thermal cooking and microwave cooking, as evidenced by Boehm U.S. Pat. No. 3,440,386 and French Pat. No. 1,249,130. However, such structures present practical problems where operated in three modes namely microwave, thermal and self-clean. Thus when the temperature in the cavity is raised to a level, on the order of 900.degree. F., capable of producing thermal decomposition of the soil on the cavity surfaces, a slightly positive pressure is created causing back-up of the heated air and volatile decomposition products into the waveguide where condensation takes place, resulting in an accumulation, or build up, of sticky residue which not only serves to contaminate the air subsequently flowing through the waveguide during normal cookery, but which can affect the transmission characteristics of the waveguide. Such accumulation is extremely difficult to remove, requiring substantially complete disassembly of the power supply housing and waveguide.
It is, accordingly, an object of the present invention to provide a common cavity oven capable of miccrowave and thermal cookery and which is in addition capable of positively and reliably expelling the decomposition products when operated at high temperature in a self-clean mode. It is a related object of the present invention to provide a common cavity oven which is sealed against escape of microwave energy when operated in the microwave mode but which nevertheless has good ventilation characteristics when operated in the thermal and self-clean modes, insuring a well defined and unidirectional ventilation path regardless of the operating temperature and which prevents backflow of the air and volatile products of decomposition into the inlet opening regardless of the temperature achieved within the cavity.
It is a more specific object to provide a ventilation system which is capable of resisting auto ignition pressures of the products of decomposition and in which unidirectional flow of ventilating air is assured even in the presence of severe soil.
It is yet another object to provide a ventilation scheme for a single cavity oven which is capable of operating in the self-clean mode but which is simple and economical avoiding use of screens, restrictions and the like which have been resorted to in prior structures in order to insure against backflow of contaminated air.
It is a general object to provide a ventilating system for a common cavity oven usable in all three modes of operation and which is safe in all three of the modes and which runs no risk of violent explosion of the products of combustion in the self-clean mode.